1. Field of the Invention
This invention relates to large diameter raise bore rock bits.
More specifically, this invention relates to large diameter raise bore rock bits with a means in which the diameter of the rock bit may be increased by attaching radially extending segments to the rock bit.
2. Description of the Prior Art
A number of large diameter drill bits with attachable segments to enlarge the diameter of the drill bit are known in the prior art. For example, U.S. Pat. No. 4,142,598, assigned to the same assignee as the present invention, discloses a large diameter drill bit that is capable of drilling various size boreholes. The bit includes a primary body that supports a plurality of yokes positioned at various radial locations thereon. A frustoconical cutter is rotatably mounted on each yoke for contacting and disintegrating earth formations, the outermost cutters forming the gage row of the bit. A second stage in which larger diameter holes may be drilled comprises a plurality of ear assemblies for attachment to the primary body and circumferential locations between the outermost primary cutters associated with the primary body assembly. Each attachable ear assembly comprises a unitary support frame having a yoke and cutter mounted thereon. Each ear assembly support frame includes a plurality of locating pads axially aligned with the center support body which are adapted to register with mating pads located on the primary body and bolts are provided to extend through the interfaced locating pads to secure the support frame to the primary body.
This invention is disadvantaged in that each of the bolts attaching the secondary assembly to the primary body are subjected directly to shear loads when pressure is applied to the outer secondary members of the expanded drill bit.
Yet another prior art U.S. Pat. No. 4,010,808, teaches a primary cutter means positioned on a bit body for disintegrating the formations out to a first radial distance from the bit axis of rotation. Secondary cutter means are adapted to be connected to the bit body and selectively located in a first position for cutting between said first radial distance and a larger second radial distance and selectively located in a subsequent position between said first radial distance and an even larger subsequent radial distance. Expandable sections are provided to be located between the secondary cutter means and the bit body for locating the secondary cutter means in the subsequent positions. The primary bit is expanded by inserting adapter plates that extend secondary cutter means radially outwardly from the bit. The expanded sections are mounted along a plane parallel with the axis of the bit and each of the attachment bolts which radially extend through the expanded plates are subjected to shear loads under operating pressures from the cutters mounted to the expanded section of the variable diameter rock bit. Thus the same disadvantage associated with the first mentioned patent is associated with the foregoing patent.
The following two patents, namely U.S. Pat. Nos. 4,177,866 and 4,194,578, describe raise boring large diameter rock bits having retractable gage cutting elements. Each of these patents teach a means to hydraulically retract the outer diameter portion of the bits so that the bit may be easily transported through the borehole after the bit has cut the borehole. These two patents also teach a means to start a large diameter hole by hydraulically extending outer segments of the bit. The extended portion may then subsequently be retracted to either remove the bit from the hole or to reduce the borehole diameter while drilling continues. These hydraulically manipulated bits are disadvantaged in that they are not rigid enough to withstand drilling pressures during drilling operations.